1. Field of the Invention
The present invention pertains to raster scan displays and, more particularly, to digitally synthesizing a gray scale for raster scan oscilloscope displays.
2. Statement of the Problem
Signal waveforms when viewed on oscilloscope displays are not always stable, single valued functions. Rather, the signals may shift in amplitude and/or time from one sweep of the screen to the next sweep. The term "sweep" herein refers to the act of displacement of an electron beam across the phosphor coating on the inside of a cathode-ray tube display causing the phosphor to glow and to emit light.
The "gray scale" effect on an analog oscilloscope display refers to the condition of the most recently drawn waveform data being the brightest waveform on the display. Waveforms from the recent past sweeps show up in various "shades of gray" depending on how long the waveform has been on the screen. The term "gray scale" herein is defined as discrete steps of brightness between light and dark. The gray scale effect occurred in analog oscilloscopes due to the behavior of the phosphor coating. The phosphor would glow at full brightness when a waveform was initially drawn, and then would fade (unless redrawn again).
The gray scale effect on analog oscilloscopes conveyed useful information to a user of the oscilloscope. The user could determine both the current value of the waveform (i.e., from the current sweep) and values it had taken on during the prior sweeps. For example, in FIG. 1 a waveform being observed in the oscilloscope may undergo a minimum amplitude A.sub.min (as shown in FIG. 1a) or a maximum amplitude A.sub.max (as shown in FIG. 1b). The resultant combined image as seen on an analog oscilloscope display is shown in FIG. 1c. Waveform 10 is the waveform from the current sweep (hence, full brightness), waveform 20 was drawn during the last sweep (hence, almost full brightness), waveform 30 was drawn from a previous sweep (hence, dimly lit). The waveforms from other sweeps are also shown and all of these waveforms taken together result in a gray scale envelope 40 collectively placing the waveforms of varying degrees of brightness within a range of amplitude values (A.sub.min -A .sub.max).
The useful information contained in FIG. 1 includes an indication to the user of the analog oscilloscope that the waveform has a current direction 50 of change as shown in FIG. 1c. The sweep frequency (i.e., the sweep repetition rate) and the oscillation by the waveform 100 between A.sub.min and A.sub.max determines what is viewed on the screen as shown by FIG. 1c. For example, if the oscillation between amplitude values occurred at a sufficiently high rate so that the phosphor did not fade to black between refreshes, the user could see the full range of values that the waveform took provided the sweep frequency was fast enough. On the other hand, if the oscillations occurred at an extremely high rate of repetition, the user would only see the most common amplitude values for the waveform. For example, if waveform 100 spent a larger percentage of its oscillation cycle time at A.sub.max then at A.sub.min, the outer edges 60 of the gray scale envelope 40 would appear brighter to the user. This is simply due to the fact that the phosphor is excited more often at the outer edges 60 and therefore maintains a higher average level of light output.
While the above was a characteristic of analog oscilloscopes, modern digitizing oscilloscopes do not provide the user with the type of gray scale information described above. Rather, modern digitizing oscilloscopes digitize the waveform into a set of discrete values which are then written into a display memory. The display memory holds an image which is copied to the actual display. The image is copied repetitively to the display and continues to be refreshed until the image changes. This is illustrated in FIG. 2.
In FIG. 2, a display memory 200 contains the binary information in the form of ones and zeros. For example, in FIG. 2, the display memory 200 has bit pattern 210 which corresponds to a display pattern 220 on a screen 230. The binary one corresponds to an illuminated dot 240. This is graphically shown by arrows 250 and 260. Hence, individual display dots (or pixels) on screen 230 are either on or off and, to the human user, there is no intermediate brightness values which correspond to provide gray scale information. A need exists to provide corresponding gray scale information to the user of a digitizing oscilloscope.
One technique utilized in digitizing oscilloscopes is a feature called "variable persistence" which attempts to provide some gray scale information. This feature allows the user to specify how long a data point should stay on screen after it has been drawn. This is simply accomplished by having the microprocessor control determine how long a bit should stay set in display memory 200, with the length of time being controlled by the user. When the preset amount of time has passed, the microprocessor then clears the bit in the display memory which has the effect of erasing the dot from the display In digitizing oscilloscopes, because the phosphor fades from full brightness to off quickly, the data points appear to the user as either being full bright on or off. The "variable persistence feature" simply increases the density (i.e., quantity) of dots fully illuminated on the screen. Hence, there is a greater dot density at the more recent locations of the waveform and a lesser dot density at the less recent locations of the waveform.
There are clear disadvantages to the "variable persistence feature. A major disadvantage is that all data points are illuminated to the same brightness under this technique. This actually hides information from the user in two ways. First, in order to see a display of the full range of values of the waveform (i.e., between A.sub.min and A.sub.max), the user of the oscilloscope generally may have to set the persistence time to a relatively long value depending on how fast the waveform changes. The result is that all data points in the waveform range are illuminated at the same level of brightness. It is not possible to visually determine the current value of the waveform or whether the waveform tends to spend more time at some locations than others. Secondly, if the user reduces the persistence time so that the current value of the waveform can be viewed, the user will not be able to view the full range of values the waveform may take. This depends on how quickly the waveform is changing. Hence, a short persistence time could eliminate part of the waveform envelope in favor of showing the current value.
Another major disadvantage of the variable persistence method is that the display tends to be covered with a spray of illuminated dots as the waveform changes shape. Because all of the waveform data points are plotted at the same light intensity, the true shape of the waveform may be difficult to determine. It is difficult to determine which set of illuminated dots relate to which trace of the waveform. A good example is a frequency modulated waveform. The waveform itself is a sine wave but its frequency changes. On current variable persistence oscilloscopes, the change from a minimum frequency sine wave to a maximum frequency sine wave creates a solid band of dots on the screen. While the envelope showing the full range of values is created, the waveform shape is difficult to determine in the band of dots.
A final disadvantage is that the waveform shown on a digital oscilloscope display does not qualitatively look like the waveform drawn on an analog oscilloscope display. It is difficult for a digital display user to extract the type of gray scale information therefrom.
A need, therefore, exists to provide digitizing oscilloscopes with the capability of providing gray scale information as found in analog oscilloscopes.